A computer system generally maintains a copy of the operational program information ("generic") and operational data information ("data base") on a first medium, which is usually a disk system. Periodically, the information on the disk is copied to a second medium, usually magnetic tape, for the purpose of updating the system or archiving the information on the disk. In stored program controlled switching systems, for example, the generic information and the data base information are copied from the switching system's disk to a magnetic tape, sent to a central location where they are de-compiled, updated, re-compiled and sent back to the switching system. Also, most systems keep an archive tape copy of the disk information so that, if the disk information is damaged because of hardware or software errors or a computer "virus", for example, the computer disk system may be restored to a known state from the tape copy.
Over the past several years, disk information storage capacity has increased dramatically. With known computer data backup arrangements, the time required to copy disk information onto tape and the magnitude of required tape storage become increasingly serious problems with increased capacity disk storage.